The present invention relates to a method of coating a granular material and an apparatus therefor. More in particular, it relates to a method of coating a granular material, which is an improved method such that a coating film of a film-forming material can be formed at a uniform thickness on the surface of the granular material, and an apparatus therefor.
In recent years, it has been desired, for example, in granular fertilizers or agricultural chemicals, to control the dissolving rate of effective ingredients contained therein more strictly in a long period of time such as from several weeks to several months. This is so intended to obtain required effects of the fertilizers or agricultural chemicals effectively for a long period of time by a few number of application or scattering, for example, once or twice through out the course of seeding, growth and harvest of plants.
For this purpose, it has been proposed to coat the surface of a granular material such as fertilizer or agricultural chemical with a polymer.
The coating method proposed so far includes, for example, (1) a method comprising blowing a granular material upwardly by a gas stream jetted out upwardly and applying a coating material on the surface of the granular material (Japanese Patent Publication (KOKOKU) Nos. 1-38102 (1989), 2-31039 (1990) and 2-44792 (1990) (2) ; a method comprising charging a granular material in a rotating drum and spraying a coating solution composed of a organic solvent solution of a thermoplastic resin while rolling the material in the drum, thereby applying the coating material on the surface thereof (Japanese Patent Publication (KOKOKU) No. 53-44339 (1988)(3)); and a method comprising repeating steps of carrying a granular material on a rotating drum having lifts disposed on an inner wall, dropping the granular material from an upper portion in a calm region, spraying a coating solution thereby applying a coating material on the surface of the granular material in the calm region, drying in a blowing region, and dropping and then landing the material (Japanese Patent Application Laid-Open (KOKAI) No. 63-4871 (1988)).
However, in any of the methods described above, a dissolving characteristic thereof varies greatly when measured for a long period of time, and this is considered to be attributable to the reasons described below. In the method (1), since excessive stresses are applied, a film thickness varies and flaws are caused to the coating film. In the method (2), since the coating solution is sprayed on the granular material under rolling in the drum, the granular material is sometimes agglomerated to each other into lumps before drying, whereby the treated granular material tends to cause unevenness in the resultant coating film and leaves flaws due to adhesion and peeling between each of the particles. The method (3) seems to be considered most preferable, but it has been found also impossible to strictly control the dissolving rate for a long period of time by the investigation made by the present inventors. The present inventors have observed the granular material obtained in this method by a microscope of high magnification and found that small flaws are present at the coating film surface or traces of adhesion and peeling are observed in various portions of the granular materiel. According to the studies of the present inventors, it has been found that the small flaws are liable to cause when the granular material falls from a considerable height and small traces of adhesion and peeling are caused between particles of the granular material due to insufficient drying unless there is a sufficient drying time (sufficient height) during falling after spraying and depositing the coating solution on the surface thereof. Accordingly, it has been found for the method (3), that high or low falling distance results in the problem.
Further, in the methods (2) and (3), sprayed coating solution promotes coagulation between particles of the granular material to increase agglomeration depending on the amount of the granular material contained in the drum and the amount of the coating solution sprayed relative to the amount of the granular material, making it further difficult to form the uniform coating film and failing to obtain an industrially usable processing efficiency.
As a result of an earnest study made for dissolving the drawbacks in the prior arts, it has been found that in a method of coating a granular material comprising steps of dropping the granular material to be coated from an upper portion, spraying a coating solution containing a film-forming material dissolved in a solvent to the granular material in the course of falling, drying the sprayed granular material and carrying the dropped granular material upwardly, by dropping the granular material in a draft and drying the granular material sprayed with the coating solution by a drying gas flowing upwardly from a lower portion to an upper portion in the draft, a coating film can be formed uniformly and with no flaws on the surface of a granular material and contents can be dissolved uniformly. On the basis of this finding, the present invention has been attained.